The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube which is improved in color uniformity over the entire screen.
In recent years, a so-called flat-face or flat-panel type of color cathode ray tube has been widely used as picture tubes for TV receivers or monitor tubes for personal computers. In addition, a color cathode ray tube in which the pitch of phosphor layers is reduced has been provided to display high-resolution images.
The glass-made envelope of such a cathode ray tube includes a panel portion having a faceplate, a neck portion, and a funnel portion which connects the panel portion and the neck portion. The interior of the glass-made envelope is in a nearly vacuum state.
Accordingly, the thickness of each portion of the glass envelope is set to a value which enables the glass envelope to withstand atmospheric pressure. The flat-face type of cathode ray tube in particular is formed in such a manner that the peripheral portion of its faceplate is larger in thickness than the central portion of the faceplate.
In such a cathode ray tube, an electron beam emitted from an electron gun impinges on a phosphor layer formed on the inner surface of the faceplate, thereby making a phosphor emit light. The portion of the faceplate on which picture elements are formed is a screen. Light radiated outward from the outer surface of the faceplate is small in attenuation in the central portion of the faceplate made of glass of small thickness, and is large in attenuation in the peripheral portion of the faceplate made of glass of large thickness. Letting Tpc and Tpa be, respectively, the optical transmissivity of the central portion of the faceplate and the optical transmissivity of the peripheral portion of the faceplate, Tpc greater than Tpa. In other words, the luminance of an image displayed on the outer surface of the faceplate becomes lower in the peripheral portion than in the central portion of the screen. In addition, in the periphery of the screen, since the weights of phosphors are smaller than those in the center of the screen, the luminance is lowered to a further extent. Japanese Patent Laid-Open No. 238481/1999 (European Patent Laid-Open NO. 0933797) is a document which discloses an art for solving this problem.
If the pitch of the holes of a black matrix (hereinafter, BM holes) for displaying a high-resolution image is made small, the BM holes become small and the luminance is lowered. Since the horizontal pitch of phosphor layers of a recent type of cathode ray tube is 0.3 mm or less, the cathode ray tube is improved in resolution, but is lowered in luminance. If the luminance is low, the range of adjustment of white balance becomes small. In particular, in a case where the thickness of the faceplate is not uniform, since the luminance is partially lowered, it becomes difficult to uniformize white-color display over the entire screen (i.e., white uniformity).
It has been considered that in such a related art cathode ray tube, BM holes for the respective colors are set so that white color can be obtained in the central portion of the screen, and BM holes positioned in the peripheral portion of the screen are made the same in diameter as BM holes positioned in the center of the screen, whereby white-color display becomes uniform over the entire screen. Japanese Patent Laid-Open No. 63480/1997 is a document which discloses this art.
However, in the case of a panel including a flat outer surface and an inner surface having a curved surface, even if the diameters of its BM holes for three colors are made uniform, there occurs the problem that the central and peripheral portions of the screen differ from each other in color temperature. In other words, there occurs the problem that white uniformity cannot be ensured over the entire screen.
The present inventor has discovered that a cause which makes it impossible to ensure white uniformity is the spectral transmittance of panel glass. That is to say, in a panel having a flat outer surface, since the difference in panel thickness between the central and peripheral portions of the panel, the influence of the difference in the spectral transmittance of the panel between the central portion and the peripheral portion becomes remarkable and degrades white uniformity.
To cope with this subject, the present invention compensates for the difference in the spectral transmittance of panel glass by causing the difference in BM hole diameter between the central portion of a screen and the peripheral portion of the screen, i.e., gradient, to differ among individual colors.